There has been considerable interest in developing high resolution, non-destructive, non-contact techniques for evaluating samples. The need is particularly acute in the semiconductor industry where feature sizes are often in the micron and submicron range.
The assignee herein has developed one such device which is marketed under the trademark Thermaprobe Imager. The operation of this device is described in the following United States Patents which are hereby incorporated by reference: U.S. Pat. Nos. 4,521,118; 4,522,510; 4,636,088; 4,579,463; 4,854,710; 4,952,063 and 5,042,952.
All of the devices described in the above cited patents include a means for periodically exciting the sample at a highly localized spot on the sample surface. In the commercial embodiment of the device, this means is defined by an intensity modulated pump beam from an argon laser which is focused to a spot size of about one micron on the sample surface. The pump beam functions to periodically heat the sample which, in turn, generates thermal waves that propagate from the irradiated spot. In a semiconductor sample, the pump beam also functions to create an electron-hole plasma which propagates in a manner analogous to a thermal wave. Both the thermal and plasma waves interact with boundaries and barriers which scatter and reflect the waves. Features at or beneath the sample surface can be studied by monitoring the variations they induce in these waves.
As described in the cited patents, the thermal or plasma waves can be detected using a non-modulated probe beam of radiation. The probe beam is focused within the area on the sample surface which has been periodically excited. In one approach, the periodic angular displacements of the reflected probe beam, which are a result of the periodic angular changes in the surface of the sample, are monitored. In an alternate approach better suited to semiconductor samples, the periodic changes in the power of the reflected probe beam, which are a result of the changes in the optical reflectivity of the sample, are monitored. A third approach would include the use of an interferometer configuration for monitoring the periodic changes in the height of the sample surface within the periodically heated region. In each of these approaches, a phase sensitive detection system is used to monitor changes which are synchronous with the modulation frequency of the pump beam.
As described in the above cited patents, when the optical reflectivity of the sample is to be monitored, it is desirable to arrange the pump and probe beams to be coincident on the sample. This configuration helps to maximize the modulated power signal. In contrast, when the angular deflection of the beam is to be monitored, the pump and probe beams are separated a short distance, on the order of one to two microns. This separation functions to maximize the deflection signal.
FIG. 1 is a graph illustrating the signal strength which is available as a function of the distance from the center of the periodically heated spot C. As can be seen from curve 10, the modulated optical reflectivity signal is at maximum at the center of the heated spot and drops off rapidly as the distance from the heated spot increases. This sensitivity pattern would be the same for an interferometric detection system used to monitor periodic changes in the height of the sample due to the periodic heating. Curve 12 illustrates the signal available when measuring the angular deviations of the reflected probe beam. This signal is essentially zero at the center of the heated spot, increases and then once again decreases as the distance from the center of the spot C increases.
The scale of FIG. 1 is dependent on factors such as the thermal conductivity, specific heat and density of the sample as well as the modulation frequency of the pump beam. For a typical semiconductor sample and a modulation frequency on the order of 1 Mhz, the maximum signal strength represented in curves 10 and 12 will be attenuated by approximately 90% at a distance of 2 microns from the center of the periodically excited spot C. Even though the periodically excited region extends beyond two microns, it did not previously appear reasonable to operate with the probe beam separated from the pump beam a distance of more than two microns since the signal strength in that region is so highly attenuated. As will be discussed below, for certain samples, separating the pump and probe beams a distance of two microns or more has some unexpected benefits.